Want to flood-proof Manhattan? It’ll cost at least $11.6 billion

More bang per buck from adaptation, but that will change with the climate.

Key landmarks are shown in yellow. Sites of potential flood barriers are in red.

John Timmer

Want to make Manhattan waterproof? A few years ago, the idea seemed ludicrous to everyone but a handful of researchers and urban planners. But the aftermath of Hurricane Sandy, extended outages, and infrastructure damage changed the conversation. Suddenly, long-ignored plans for multi-billion-dollar flood barricades were given serious attention.

But does it merit the attention? In today's issue of Science, researchers describe a risk assessment model that they use to consider a variety of approaches for flood-proofing the Big Apple. They conclude that, for now, adaptation makes more economic sense than a giant flood barrier—but that may change. Under a middle-of-the-road climate scenario, where storm intensity and ocean levels both rise, building a barrier starts to make sense before mid-century.

Options

The authors of the new study (who are scattered among four different academic institutions) focus on a number of different plans for minimizing the impact of floods. At the low end, the city would simply harden key pieces of infrastructure, like the subways, airports, and electrical generation and distribution sites. Accompanying that would be changes in the building codes requiring that buildings elevate above common flood levels and/or incorporate basic levels of waterproofing. Since New York has a massive existing stock of buildings, some effort would also have to be made to compel changes in that.

Beyond this, however, you have to start building barriers. And here, the complicated geography of New York poses a number of challenges. The inner harbor (the part that borders Wall Street and the Statue of Liberty) actually has three entrances: the gap between Brooklyn and Staten Island to the southeast, the channel between Staten Island and New Jersey to the south, and the passage between Queens and the Bronx where Long Island Sound meets the East River. To fully protect everything that faces the inner harbor, including all of Manhattan, you need barriers at each of those entrances.

But these barriers would leave large parts of the city unprotected, including the portions of Staten Island that face the outer harbor. Jamaica Bay, a large body of water near Kennedy Airport, also poses a risk for large areas of Brooklyn and Queens. It's possible to place a barrier at its entrance as well, although this would require building up the barrier islands between Jamaica Bay and the Atlantic Ocean.

If you're doing that much construction, however, it's possible to connect New York's barrier islands to New Jersey and cordon off the outer harbor too. This would obviate the need for any other barriers with the exception of the one between Queens and the Bronx in the north. The New Jersey side, a peninsula called Sandy Hook, would require considerable construction as well, since it's (as its name implies) largely a pile of sand with boundaries that shift slightly with large storms.

Counting the costs

All of these approaches can be done with existing technology, so the authors turn to cost-benefit analysis. Using a framework they've developed (one that they note could be applied to any other coastal city) they calculated the economic costs of flooding in New York. The average annual cost comes out to be $174 million, but the contributions of rare events are catastrophic. A 100-year flood will come in at $2.2 billion, while a 1,000-year flood would create $25 billion in damages.

But even the cheapest plan—changing building codes and protecting key infrastructure—would cost $11.6 billion. Building three barriers ups the bill to $14.7 billion, while sealing off the entire harbor comes in at nearly $24 billion. The annual costs for maintaining the barricade solutions are also considerable, although critical. For the purposes of this analysis, "Flood defenses in the storm surge barrier strategies are assumed not to fail."

Compared to the annual damages the authors calculated, none of the barrier solutions makes economic sense at the moment. However, if the effects of climate change are stronger than what we see in the most probable scenarios considered by the IPCC, then the cost-benefit analysis starts to favor the barriers. By 2040, a middle-of-the-road climate scenario starts to make sense.

Although the costs are clearly extensive, the authors point out that there are a lot of people with a direct interest in keeping anything like Sandy from happening again. Even the most limited barriers would protect large areas of New Jersey, with the area protected growing significantly if a barrier is built to Sandy Hook. In addition, the disruption in economic activity after Sandy was felt well beyond the US; it's possible to make some of the project a public-private partnership. The authors also note that a $10 fee on each tourist would generate half a billion dollars a year.

As of now, New York City has already started the process of protecting key infrastructure, part of the approach that the study considers the most cost-effective for the moment. And indications are that we have several decades before more radical steps need to be taken. Still, given the number of parties that will need to be involved in building barriers, 25 years of lead time might be essential for a project of that scale.